Yes. You need to set up a shared variable in the subreport like:
whileprintingrecords;
shared numbervar sumsub := sum({table.amt});
You have to place the formula in the subreport report footer and then create another formula in the main report that references it, as in:
whileprintingrecords;
shared numbervar sumsub;
sumsub + sum({mainrpt.amt})
The above calculation formula would need to be placed in a section below the one in which the subreport was executing. You probably need to share what your main report groups are, how you are linking the sub to the main report, and where the sub is located in the main report.
-LB